Bacterial Pathogenesis 1 & 2 Slides PDF

Bacterial Pathogenesis I & II Dr. Raj Department of Microbiology College of Medicine, IAU [email protected]  Learning Objectives After attending this lecture, you are expected to know  Host factors that help to prevent bacterial infection  The mechanisms of bacterial invasion of...

Bacterial Pathogenesis I & II Dr. Raj Department of Microbiology College of Medicine, IAU [email protected]  Learning Objectives After attending this lecture, you are expected to know  Host factors that help to prevent bacterial infection  The mechanisms of bacterial invasion of human body  How bacteria overcome host defenses.  Various virulence factors that play role in pathogenesis.  Endotoxins and exotoxins and their role in pathogenesis 2  5:27 PM Introduction  Different types of infection  Bacterial pathogenesis is process of disease production by bacteria  It depends on so many factors of agent, host and environment  Pathogenicity - refer to the ability of a microbial species to produce disease.  Virulence - relative degree of pathogenesis (tissue damage), which may vary between different strains of the same organism depending upon the expression of the virulence factors. 3  5:27 PM Different types of Infection Primary Initial (first) infection by a pathogen with new host infection Reinfection Subsequent infection by the same pathogen in same host Secondary New infection by another pathogen due to immunodeficiency caused by primary infection infection Ex., Staph aureus or pneumococcal infection in a influenza patient Focal Localized infection in some organs or tissues (tonsils, appendix, dental abscess, liver infection abscess, etc) generalized effects occur systemically (Focal sepsis) Cross New infection from another host or external source in a patient who is already suffering from infection a disease, most commonly seen in hospitals from others 4  Different types of infection Iatrogenic infection Physician (doctor) induced infections (treatment, diagnosis or investigations or any other procedures) Exogenous OR Source of infection is from external sources Endogenous infection Source of infection is from hosts own body Inapparent infection There is an infection but clinical symptoms are not apparent or not manifested (sub clinical) Atypical infection Typical or classical or characteristic clinical features of disease are absent Nosocomial infection Hospital acquired infections, during stay or after admission or after discharge from hospital  Different types of infection Latent infection Some pathogens after primary infection remain in tissues in a hidden or dormant or latent stage from proliferating and later when host resistance is lowered multiply to produce disease Co-infection Simultaneous infection of a host by multiple pathogens, ex HDV & HBV together, multi- parasite infections, HBV & HCV in blood transfusion, etc Bacteremia Circulation or presence of bacteria in blood Septicemia Bacteria circulate and multiply in blood, form toxic products and cause high swinging type of fever  5:27 PM Medical terms Minimum Infective Dose (MID) minimum inoculum size that can initiate an infection or minimum number of bacteria required to produce clinical evidence of infection in a susceptible host under standard conditions Minimum lethal Dose (MLD) minimum number of bacteria required to produce death of a host Normal flora microorganisms (bacteria) that continuously inhabit (live) the human body and live together without harming 7 Lower Limb Overview  How Do Bacteria Cause Disease ?? 1. Transmission from exogenous sources or infected individuals – Epidemiological triad 2. Entry into human body 3. Overcome host defenses 4. Multiply and disseminate within the body - Adhere to host surfaces - Invade host cells 5. Cause cell and tissue damage 6. Spread to other members of population 9  The 'Iceberg' Concept of Infectious Disease Many infections are asymptomatic in many individuals Illness can be mild, moderate or severe Asymptomatically infected individuals are important because,  they develop immunity and resistance to reinfection,  they are not identified,  move normally in the community and can infect others.  The 'Iceberg' Concept of Infectious Disease What are the 3 sections of the "iceberg" concept of infectious disease ? 1. Classical Clinical Disease (the very tip) 2. Less severe disease (base of the tip) 3. Asymptomatic infection (Hidden- the largest) Example – Polio for every 1 case thousands of cases as asymptomatic or infected in a community. The 'iceberg' concept of infectious disease. 12  Host factors that help to prevent bacterial infection Physical & chemical barriers  High salt, fatty acids (skin)  Acid (stomach)  Mucous + cilia (Lower Resp Tract)  Peristalsis (Gastrointestinal Tract)  Soluble mediators eg. Lysozyme Innate immune responses  Complement  Phagocytosis Adaptive immune responses  Antibodies by B cells (IgG, A M, D, E)  Cell-mediated immunity (cytotoxic T-cells, T helper cells) 13 5:27 PM 14  Normal flora can cause disease Bacteria inhabit a wide variety of different niches on human body  Skin  Respiratory tract  Gastrointestinal tract (mouth to anus)  Urinogenital tract NORMALLY THESE BACTERIA DO NOT CAUSE DISEASE BUT – they can cause disease (opportunistic infections) if:  they gain entry to the body cavity  there is a change in the ecological niche  the host is immunosuppressed 15  Bacterial virulence factors - IMPORTANT SLIDE Virulent Factor Function/Role Pili/fimbriae/adhesins Adhesion Toxins Tissue Damage/Modulate host immune response Lipopolysaccharide Modulate host immune response/ tissue Damage Invasins Invasion / Modify Host response Flagella Motility / chemotaxis Polysaccharide capsules Anti-phagocytosis and adhesion Enzymes - coagulase, Facilitate organism survival, multiplication, act on host tissues, prevent host hyaluronidase, lecithinase defense Iron binding proteins iron chelators produced by bacteria that bind iron with an extremely high affinity (Siderophores) and can remove iron from host proteins, for transport into the bacterial cell 16  Adhesion Colonisation usually involves adherence to host cells Adherence to host cells mediated by external structures called adhesins They can be either: A. Pili or fimbriae B. Non-fimbrial adhesins as Capsule or protein structure on the cell wall 17  A. Pili or Fimbriae Short, hair-like structures on the cell surface Tip of pilus mediates adhesion Tip confers host specificity Adherence to surfaces, which facilitates infection, and is a key virulence feature Resist phagocytosis by Polymorphonuclear leukocytes Example  Neisseria gonorrhea - gonorrhea  Escherichia coli – UTI, diarrhea 18 Another example - Enterotoxigenic Escherichia coli (ETEC) Most common cause of travelers' diarrhea Use antigenically specific fimbriae/pili Called Colonization Factor Antigens (CFA) 19  B. Non-Fimbrial Adhesins Eg. Bordetella pertussis Cause of whooping cough Filamentous haemagglutinin (FHA) Mediates adhesion to ciliated epithelial cells and are essential for tracheal colonization (binds to host B1 integrins) Host cell surface protein or carbohydrate 20  Adhesins helps in Biofilm formation Biofilm is a bacterial aggregates on mucosal or oral or foreign body surfaces To form biofilm bacteria must have  Innate resistance to host immune defenses  Increased tolerance to stresses, including starvation and dehydration  Increased tolerance to antimicrobial agents (10 –1,000 times more than normal) Example: Staphylococcus epidermidis infections are associated with biofilm formation in implanted devices: e.g.: joint prosthesis, heart valve, shunts, and intravenous catheters 21  Capsule or Slime A slime layer is loosely associated with the bacterium and can be easily washed off, whereas a capsule is attached tightly to the bacterium and has definite boundaries. Usually made of polysaccharide – many bacteria Made up of Protein in Anthrax bacillus Immune evasion - protect a bacterial cell from ingestion and destruction by white blood cells (phagocytosis), inhibit binding of complement Allow bacteria to stick to each other and to surface eg. implanted devices. 22  5:27 PM Capsules  Responsible for the mucoid appearance of bacterial colonies  Demonstrate under light microscope by India ink stain  Capsule appear as clear halos surrounding the bacterial cells  Useful in presumptive diagnosis  Examples – Streptococcus pneumoniae – Klebsiella pneumoniae – Neisseria meningitidis 23 Lower Limb Overview  5:27 PM Enzymes as virulence factors  Hyaluronidase- breaks down the connective tissue component hyaluronic acid  Hemolysins(Staphylococci & Streptococci) - breaks down red blood cells  Coagulase – fibrin barrier around organisms – Staph aureus  Streptokinase (fibrinolysin) Strep pyogenes, breaks down fibrin barrier, helps spreading  DNases -break down DNA(depolymerization), Staph aureus  Proteases and lipases - breakdown proteins and lipids  Collagenase – digest and lyse host collagen tissue, helps infection spread  Lecithinase -phospholipase that breaks lecithin, Clostridial species, myonecrosis and hemolysis  Urease - Proteus in urinary tract and H. pylori in stomach, splits urea to ammonia to CO 2, creates alkaline Ph, survival advantage 24 Lower Limb Overview  Bacterial Toxins Bacteria produce Two Types of Toxins Endotoxin Exotoxin  6:53 PM Endotoxins Integral parts of the cell wall and are normally released only when the cell dies. Endotoxins are particularly characteristic of Gram-negative bacteria. Endotoxins are typically lipopolysaccharides in nature Coded by bacterial chromosome 26  6:53 PM Role of Endotoxin in Pathogenesis  Endotoxin activates a number of pathways:  1. Activates large scale cytokine release (TNF-a, IL-1, IL-6 )  2. Activation of clotting pathways  3. Activation of complement pathways  Leads to fever and inflammation  So the effect of the endotoxin is indirect by inducing the immune response 27  6:53 PM Endotoxic (Septic) Shock Consequence of exposure to high levels of endotoxin Most commonly seen with: gram negative bacteria ✓ Escherichia coli and other Enterobacteriaceae ✓ Pseudomonas aeruginosa ✓ Neisseria meningitidis 28  6:53 PM Systemic Effects of High Doses of Endotoxin Fever Hypotension Shock Weakness Blood coagulation (DIC =disseminated intravascular coagulation) Inflammation Intestinal hemorrhage 29  6:53 PM Many activities of bacterial endotoxin. Lipopolysaccharide (LPS) and, as a result, LPS is one of the most powerful immune stimactivates almost every immune mechanism as well as the clotting pathwayulus known. LBP- LPS binding protein 30  Exotoxins Heat labile proteins generally secreted by bacteria (G+VE) Diffuse readily into surrounding medium Highly potent even in small amounts Usually composed of two or more subunits Cause damage to the host by destroying cells or disrupting normal cellular metabolism Exhibit tissue specificity  Exotoxins Coded by Plasmid & Bacteriophage Can be toxoided, vaccine production Some exotoxins act as superantigens (massive T-lymphocyte activation) Staph aureus, Strep pyogenes, Clostridium tetani, Clostridium botulinum, Corynebacterium diphtheriae, Bordetella pertusis E.coli (diarrheagenic),Vibrio cholerae, Shigella, Pseudomonas 33 34  Pathological effects of infection – general scheme 1 2 Bacteria can cause tissue damage either: 3 1. Directly through exotoxins 2. Indirectly through innate or adaptive immune mechanisms or inflammation activated by 4 endotoxins Mims' Medical Microbiology and Immunology, 18, 168-186  Effects of Sepsis 36 MODS-Multi organ dysfunction syndrome  6:53 PM Major Classes of Bacterial Exotoxins  Based on Mechanism of Action 1. Toxins that act directly at the target cell surface.  Alpha-toxin and Perfringolysin O produced by Clostridium perfringens 2. Toxins that modify intracellular target molecules.  diphtheria toxin, cholera toxin 3. Toxins that act on neuromuscular junction (nerve-muscle action)  Tetanospasmin, Botulinum toxin 37  6:53 PM 1. Toxins Acting at the Target Cell Surface 1. Clostridium perfringens (C. perfringens) - alpha toxin (α) (Lecithinase) 2. C. perfringens perfringolysin 0 - theta toxin (θ) 38  6:53 PM 2. Toxins That Modify Intracellular Target Molecules Most common type of bacterial toxin. Usually have A & B subunits, where ✓ A subunit - has enzymic activity, modifies a specific intracellular target. ✓ B subunit - responsible for binding of toxin to cell membrane and translocation of A component into cell. Two distinct groups ✓ Simple A-B toxins eg. Diphtheria toxin ✓ Compound A-B toxins eg. Cholera toxin (AB5) 39  6:53 PM Diphtheria toxin Cholera toxin Salyers & Whitt Fig 4.1A 40  6:53 PM Diphtheria toxin Exotoxin secreted by Corynebacterium diphtheriae ( G +ve aerobic bacilli) Disease caused by C. diphtheriae is called as Diphtheria Diphtheria is a fatal infection of the upper respiratory tract (tonsils, pharynx, larynx ) The toxin causes ulceration, extensive inflammation and swelling. Necrosis of the epithelial cells leads to the formation of Pseudo membrane “false membrane” & Obstruction of respiratory tract. Virulent strains carry a lysogenic bacteriophage (a virus that infects bacteria, incorporates its DNA into the host cell nucleus and replicates with it without destroying the cell) that encodes diphtheria toxin. Prevented by vaccination. DTaP (Diphtheria, Tetanus, Pertussis) Vaccine 41  6:53 PM Mode of Action of Diphtheria Toxin 42  6:53 PM Cholera Toxin Cholera is an acute diarrheal infection caused by ingestion of food or water contaminated with the bacterium Vibrio cholerae Vibrio cholerae - comma shaped G-negative rods. Cholera Toxin causes excessive secretion of electrolytes and water into gut lumen Cholera toxin causes fluid accumulation in gut leading to watery diarrhea. Death results from loss of fluids and electrolytes. Cholera toxin is encoded by two separate genes located on bacteriophage. Cholera toxin is a compound AB5 toxin. 43  6:53 PM Mode of Action of Cholera Toxin The B subunit binds to the cell receptor and internalize the A (A1 and A2) subunit to the cell. The A1 subunit activates the formation of cAMP by the adenylate cyclase enzyme. The accumulation of cAMP in the cytoplasm leads to transportation of ions and water out of the cells Causes accumulation of water in the Intestine and severe diarrhea Diarrhea caused by cholera- Rice watery diarrhea Massive loss of fluids and electrolytes - hypotension 44  6:53 PM 3. Toxins That Work on Nerve-Muscle Transmission a. Tetanospasmin produced by Clostridium tetani b. Botulinum toxin produced by Clostridium botulinum 45  Tetanospasmin (Tetanus neurotoxin (TeNT)) Clostridium tetani is a gram-positive spore forming anaerobic bacillus. Produces two plasmid coded toxins, tetanospasmin, the neurotoxic component and tetanolysin, a hemolysin It causes a disease called tetanus (spastic paralysis, excessive contraction of skeletal muscles) The toxin is very potent: the lethal dose is 2.5 ng/kg. The toxin blocks the release of the presynaptic inhibitory transmitters leading to continuous stimulation & contraction of skeletal muscles Inhibitory neurotransmitters - Glycine and Gamma-aminobutyric acid (GABA)  6:53 PM Tetanospasmin (Tetanus neurotoxin (TeNT)) 47  6:53 PM Botulinum toxin Is a exotoxin produced by Clostridium botulinum Clostridium botulinum is a gram-positive spore forming aerobic rod. It causes a disease called Botulism (flaccid paralysis, excessive relaxation of skeletal muscles) The toxin blocks the release of the acetylcholine (ACh) transmitters leading to unresponsiveness of the muscles Acetylcholine (ACh) is a stimulatory or excitatory neurotransmitter, causes contraction of muscles 48  Summary Host factors that help to prevent bacterial infection The mechanisms of bacterial invasion of human body How bacteria overcome host defenses. Various virulence factors that play role in pathogenesis. Endotoxins and exotoxins and their role in pathogenesis  5:27 PM Questions Thank you 50 Lower Limb Overview  5:27 PM Further reading (reference)  MIMS textbook of medical microbiology 7th edition 51 Lower Limb Overview  5:27 PM SAQ for practice  Name any 4 Bacterial virulence factors  What are the two types of bacterial toxins  What are the systemic effects of high doses of endotoxin  Write any 4 difference between exotoxins and endotoxins  What is the main component of endotoxin  What is the mechanism of action of tetanus and botulinum toxin  What is cholera 52 Lower Limb Overview